Transmission-level-control system



R. K. POTTER TRANSMISSION LEVEL 'CONTROL SYSTEM Filed Deo.

INVENTOR /vtel' A TTORNEY s. lli

Patenteclet; N27..

ir rare naar osic` RALPH iz. POTTER, or NEW YORK, N. i., AssIGNoR To AMERICAN TELErHoNE AND TELEGRAPECOMPANY, A CORPORATION or NEW YORK.

. lTHAN'SIYlISSIOT-IillijVELCONTROL SYSTEM.

- VApplication iled December 2., 1925. Serial No. 72,841.

This invention relates to transmission level control systems, and particularly to a system of that type in which the signal-to-nois'e ratio in transmission vbetween stations is automaticallyl maintained substantially constant.

A two-way radio transmission system in which the terminal stations are connected to wire circuits is equivalent to a four-wire repeater in which the connection to'the Wire.

circuits is established by hybrid coils. In such a radio system, it is desirable at times to raise the transmission level at the station which `is transmitting, at any instant, in order that it will be sufiiciently above the noise level of the transmission path, so that the signal may be clearly detected at the receiving end. Since such a radio system is equivalent to a four-wire repeater circuit, it is, of course, necessary to make a corresponding reduction in the amplification at the receiving end in order to prevent singing over the circuit. In systems heretofore proposed, this control of the transmitting and receiving signal levels has been .effected manually, that 'is to say, by adjustment of the' apparatus by an attendant at the transmitting and receiving stations.

It is the object of the present invention to automatically control-the transmitting and` Vreceiving levels of a two-way radio system.

Other objects of this invention will be apparent from the following description when read in connection with -the attached ldrawing, of which Figure 1 represents one of the stations of a system embodyingthe invention; Fig. 2, which is a duplication of Fig. 1, represents another station of the said system; and Fig. 8 shows the characteristic of' a filter used in the system.

In Fig. 1, which will be designated station A of the system, the wire line Ll represents a. circuit by means of which signaling apparatus, such as a subscribers' telephoneor telegraph apparatus, may be connected. This line is connected with the radio transmitting and receiving circuits of station A by means of the hybrid coil 1. Connected withthe winding of this coil, to which the line L, is

connected, is an artificial line 2 simulating electrically the line L1. A third winding of the coil l is connected' with the modulatoi` M1v of the transmitting branch of the radio circuit'.` lNhile this modulator may be of any well-known type, a preferable form is that disclosed in the copending application of Potter, Serial No. 70,405, filed November 20,

1925, in which the carrieroscillations are" suppressed. The output of the modulator is connected with the filter F1, which is of a.A type adapted to transmit only one of the side bands resulting from modulation. The output side of the filter F1 is connected with a regulator, namely, the vacuum tube 3, the

function of which is to control the amplitude of the side band impressed thereon for transmission from the station.- This regulator is coupled to the power amplifier 4, to the input side of which is also connected, by means of a transformer 5, avcircuit' over which a control signal is transmitted. The

R1 of station B shown in Fig. 2, which will be later described in` detail. The carrier oscillations which are modulated in the device .M1`are supplied thereto by meansof `the oscillator 7.

Connected Awith the receiving antenna R1 of the system are two circuits designated 8 and 9. Circuit 8 is connected with the input nature of this control signal and its method I lside of the amplifier 10, which has its output side connected .with the detector 11, whichv detector may be of any well-known type. The output side of the detector is connected with' the filterF, which is of a type adapted to pass into the circuit l2 the side band representing the communication signal. Circuit 12 is bridged across the midpoints of the line windings of the hybrid c oil l, by virtue of which the communication signal passes to the said line for transmission thereover to the distant subscriber. The filter F., is also adapted to impress upon the circuit 13a control signal. This circuit has connected therewith a rectifier comprising the tubes 14 and 15, by means of which the control signal is rectified, and, by virtue of its connection with the regulator 3 over jthe path traversed `communication 'signal represented by the4 band f-l-s, but to permit the passage therethroughof frequencies lying on each side of the "communication signal frequencies. Thus, for example, disturbing frequencies which are present in the path traversed by `the communication signal, and which are sufficiently close thereto in the frequency spectrum in order to vinterfere with the proper reception of the communication signal, are transmitted through the filter F3, in order to control, in a manner which will now be made clear, communication signal to the noise level in by thev communication signal. Theoutput side of the filter F3 vis connected with a rectifier 17 which'is coupled with the amplifier 18. In of this amplifier is a thermocouple 19 which is of the well-known type, which impresses a direct current potential upon thev grid of the amplifier 20 proportional to the current flowing through the heating element of the thermocouple 19.

amplifier 2O is connected with the input side of the amplifier 10, being bridged across the resistance 21, 'and itis also connected with the input side of the amplifier 22.` The connection of the amplifier 2O with the amplifiers 10 and 22 is made insuch way that the action of the amplifier 2O will tend to simultaneously increase or simultaneously decrease the amplification of the amplifiers 10 and 22. The oscillator? which, as stated, is

y connected to the modulator M1 is'also lcoupled bythe transformer 23 with the modulator 24, to the input circuit of which is Aalso connected, by means of the transformer 25, a source of oscillations 26, known as the 'conl trol signal oscillator. The frequency of 'the oscillations produced by this oscillator would as to lie outside the band 1 preferably be such of frequencies constituting the communication signal. As shownv on the drawing, it may be of the order of 10,00() cycles but this, of course, is purely illustrative. The output'of the modulator 24 isconnected with vthe filter 27 .which is of a type intended to suppress the carrier frequency and one of the side bands, and the output side of this filter is connected by the transformer 28 with the input side of the amplifier 22. The out-put side of the amplifier 22 vis connected the circuit 6 withA the input side ofthe power amplifierin the communication-transmission branch of the terminal circuit.

The other station designated B of the syslthe relative level of the the output circuitv The output circuit of they through the ilter tem, which is shown in Fig., I2, is preferably similar to station A. In Fig. 2, thesame reference characters have been used as in Fig. 1, except that prime marlcs have been used to distinguish the similar parts in the latter figure. y y The inventionwill be clearly understood by first tracing the paths `traversed in transmitting a signal from the line L1, to the line L, of Fig. 2, and in tracing the reception by the line L1 of Fig. 1 of a. communication signal transmittedfrom the line L, of Fig. 2. Then the method of generating and transmitting and receiving the control signal willv be fully described, and finally, the mannerl B in which the control signal circuit operates to automatically maintain constant the ratio 'l between the signal level and the noise'levelf' throughouta system/such asrepresented by stations Agand B. .A

When a communication signal which, for sake of brevity, will be referred to'as a speech signal, is impressed upon the line L, by a telephone connected therewith, the current resulting therefrom 'will flow over the line- L1 and through lthe hybrid;i coil windings, and will be impressed uponlthe modulator. M1 in the transmitting branch of station A. This speech band willi modulate carrier oscillations from the' source 7; One

of the bands resulting from such modulation will be selected by the filter F1. and will be impressed upon the regulator 3. This regulator,

which is controlled through the rectifier-amplifier tubes 14.-i5, I'would normally be adjusted so as to permit therethrough a side band having a fixed amplitude. This vside band would pass to the power amplifier 4, and after having given the required degree of amplification, would fbe 'radiated from the antenna T1 .to the receiving antenna. R1 at station B. The speechv band `will pass 4into the receiving circuit 8 and 'upon being u would be detected by 11'.; ,of F2 into the ,'cir/ eu'it 12', .l and thence' wouldbe transmitted over the ".5 -lme L1 to the subscriber atthe distant end u amplified by 10', I The detected speech band would then pass I vof that line. what this Speech band is being traag-1 mitted from station A to station B, a con-v trol signalis being transmitted simultane-f l ously from station B to station A. This control signal is produced 'by 'modulating' the carrier frequency from the-source 7. by a control signal ofa different frequency,-

l'lll Hin for example. of 10,000 cyclesvproduced by the source 26. This lmodulation is effected by the modulator 24', and after one of the side bandsand the carrier have been sup-J pressed by the lilter27, the resultant side band is impressed by thetransfor'mer 28 upon the amplifier 22'.' The amplication of this control signal depends ,upon attenu- ,3

ation and interference in .the transmitting path between station B and station A, of which further will be said hereinafter. The control signal as amplifiedby 22 is c0nducted over the circuit 6 to the input side of the power amplifier 4', and is radiated from the transmitter T1'v to the receiving antenna R1 of station A. This control signal passes into the circuit 8 and is amplified by 10 and the 10,000 cycle component is detected by 1l. The filter F2 passes this control signal into the circuit 13 wherein it is rectified by the rectifier 14 15 and controls the output of the r-egulator 3, that is to say, the amplitude of the speech Wave lbeing transmitted by the antenna T1.

if we are transmitting speech signals from station A to station B, and, of course, receiving a controlsignal simultaneously from station B at station A, itis necessary to adjust the regulator 3 at the outset in order to give the required signallevel for what may be termed the normal noise level in the path traversed both by the speech signal and the control signal. It will, of course, be apparent that since both the speech signal and the control signal traverse the same path,

vthey will be affected to substantially the same extent, and therefore, the variation in the intensity of one is in a measure the variation in the intensity of the other. Ac cordingly, while speech signals are. transmitted from the `line L, through the modulator M1, the regulator 3. the amplifier 4 and the antenna T1 to the antenna R,,

` ,thence over the receiving circuit of station B,

including the amplifier 10', the detector 11', the filter F2', to the line L1', the regulator 3 at station A is adjusted in order to give the required degree of amplification to the speech signal, which adjustment is effected bythe regulation ofthe rectifier 1415 at station A, and also by adjustment o-f the amplifier 22' at station B.l After this adjustment has been effected, and assuming that the interference level and attenuation over the transmission path remain the same between the two stations, the incoming control signal at station A, which 'is impressed upon the circuit 13 by the filter F2, will not changethe amplitude of the outgoing control signal. If the attenuation of the communication signal between stations A and B is increased, lthe attenuation of the control signal between B and A will'likewise be increased, so that a yweaker control signal is received by station A. This control signal whenrectified by 14 and impressed on 15 will cause the regulator-3 to in crease the amplitude of the outgoing communication signal to station B, which insures vthe receipt of a strong signal at stafion B. If, on the other hand, the attenuation of the communication signal would be less than normal between A and B` so that the speech signal would be too strong, the

through the filter F3', and when rectified by 17' and amplified' by 18', will set up a voltage across the potential terminals of the thermocouple 19', which voltage affects the amplifier 20. The effect of this amplifier is to increase or decrease the gain of t-he amplifiers 10 and 22', affecting both amplifiers in the same way. Let it be assumed that the noise level as represented by f is higher than normal. This would require; an increase in the communication signal sent from A. To effect this result it is necessary to reduce the control signal sent from B. The control signal being produced at station B and passing through the amplifier 22' will have smaller amplitude than normal due to rectified noise, v

and consequently this wave will be weaker than normal when received atv station A. This situation is similar to the receipt of a control signal having greater attenuation than normal. Therectified voltage of this contrpl signal producedby the rectifier 14 at station A will be similarly decreased.

The effect of this when impressed on the amplifier 15 is to increase the amplitude of the speech band passing through the regulator 3 to the antenna T1 and thence to the distant station B. At station B, the amplifier 10 has, as noted above, been adjusted to decrease the amplification so as to keep the overall equivalent of the transmission circuit constant and prevent singing. The re` sult of the action of the interfering frequency that passes through the filter F3V intoy what may be termed a supplemental control circuit is`to raise the transmission level of the signal radiated by the antenna T1 so that it will be above the increased noise level in the path between stations A and B. Interfering impulses taken from either side of the signal channel are thereby directed into the rectifying tube, which might actually be sensitized by the additional application of a l constant level local oscillation. The disturbances in the output of this tube are then impressed upon a circuit which responds slowly to changes in the magnitude of input voltage variations. As shown in the circuit. the device employed is a thermocouple with heater element in series with the plate circuit of onev tube and the voltage generator` so as to place afbiasing voltage on the grid of the subsequent vacuum tube. When properly noise level may be maintained constant be- Y although only that control signal is employedwhich is passing in a direction opposite toy `Whilethis invention Consists 1n :sisl

tween the stations of the system.

lVhen speech signals are transmitted from.

station B tostation A, a control signal passes from station A to station" B, and serves to control the amplitude of the speech signal sent out by the antenna T1 in the same manner as described above. It is important to point out vention is disclosed, the control signal is being radiated from both stations at all times,

that in which the speech signal 1s going. An arrangement for ysuppressing thc`transmission of the control signal from the same station from which at the same instant a speech signal is passing, is disclosed in my copending application, Serial No. 72,842, filed December 2, 1925, and renewed November.13,1926. f y p has been disclosed as embodied in a particular form, it is obviousll capable of embodiment in other and diferent' forms Without departing from the spirit and scope of the .appended claims.

What is claimed is: 1.- In asystem for controlling the transmission level between stations of a signaling system, the method forautomatically regulating the signal-to-noise ratio Which consists in transmitting a communication sig nal from one station to a second station of the system, simultaneously transmitting from the second`to the first station a control signal Whose lfrequency is outside the range of the communication signal, the said signal traversing substantially the same path soas to be subject to the lsame interfering sources,

and automatically controlling the amplitude of the communication signal transmitted by the said first station by the control signal received at the said first station. y

'2. In a `system for controlling the transmission level between stations o f a signaling system, the method for automatically regulating the signal-to-noise ratio which transmitting a communication signal from one station `to a second station of the system, simultaneously transmitting a control signal from the said second station to the said first station. the frequency ofthe said control signal differing from that of the said communication signal, rectify-,ying the said control signahat the 'said first station, and automatically controlling the amplitde of .the communication signal being transmitted from the said first station by the rectified control signal.

3. In a system for controlling the transmission level betivee'n stations of a signaling system, the method for automatically regulatingthe signal-'to-noise ratio which consists in transmitting a communication signal that in the form 1n which this inregulating the signal-to-noise ratio,

ously receiving at the said second station an i interfering frequency or band of frequencies lying outside therange of frequencies 'constituting the communication signal, varying the amplitude of the control signal profrom that of fromlone station to a second station of the duced atthe second stat-ion in accordance with the magnitude of the interferingfrequency or band of frequencies, rectifying the said control frequency at the said first station, andv cont-rolling the amplitude of.

the communication signal being transmitted from the said first stat-ion byl vand in accordance With the magnitude ofthe rectified control signal.

4. In a system for controlling the transmissionlcvel between stat-ions of a signaling system, the method for automatically regulatingV the signal-to-noise ratio Which consists in transmitting a communication signal from one station to a second station of the system,simultaneously transmitting a control signal'from the said second station to the said first. station, the frequency of the said control signal differing fromthat of t-hc said communication signal, varying the amplitude of the transmitted .control signal by and in accordance with tne magnitude of the interference' existing at the receiving station, and controlling the amplitude of the communicationsignal transmitted from the said first station by the instantaneous amplif tude of the control signal. y f

5L In a system for controlling the transmission level between stations of asignaling system, the method for automatically regulating the signal-to-noise ratio which consists in transmitting a, communication signal from one station to a second station of the the interferencefexisting at or 'near the said second station, receiving atftlic' first station the control signal as thus varied at the said second station, rectifying the received control signal and controlling the amplitude of the communication signal by and in accordance, With the instantaneous amplitude of the rectified control signal.

6.- In a system for controlling the transmission level between stations of, a signaling system, the method f for automaticall Whicii consists in transmitting a communication Wave from one station of the system, receiv- 'ing ata second station the said communication wave and also an interfering wave, sepa.- rating by frequencyselection the communication Wave from the interfering Wave,`

transmitting from the second station to the first station a control wave, the frequency of which lies outside the band of the communication wave, and governing the amplitude of'the control wav-e by the strength of the interfering wave. y i

7. In a system for controlling the transmission level between stations ofl a signalingsystem, the method for automatically regulating the signal-to-noiseratio, which consists 11x transmitting' a communication wave from one stationof the system, receiving at a second station 'the' said communication wave and also an interfering wave, separating by frequency selection `the communication wave from the interfering wave, amplifying thecommunication wave as'thus selected, transmitting from the second stav tion to the first station .a 4control wave, the.

frequency of. which lies outside the band of the communication wave, governing simultaneously the amplitude of the control wave and also the degree of amplification of the selected communication wave by and in ac` a second station .the said communication wave and also an mterfering wave, separating by frequency selection the communication wave from .the interfering Wave, transmitting from the second station to the first station a control wave, the frequency of which lies outside the band of the communication wave, governing the amplitude of lthe control 'wave by the strength of the in-` terfering wave, receivmg and rectifying the said control wave at the first mentioned sta- .tion and regulating the amplitude of the communication wave to be transmitted inversely with changes in the rectified control wave.

9. Ina system for controlling the transmission level between stations of a signaling system, the method forl automatically regulating the signal-to-noise ratio, which consists in generating at lone station of the system a carrier frequency, modulating the said carrier frequency by a signal frequency and transmitting the signal modulated carrier, receiving at another station the saidA signal modulated carrier, generating at the said other station a carrier frequency, modulating said carrier by a control frequency' lying outside the range of the signal frequency, transmitting the said control modulated carrier from the second station to the first station, detecting the said control fiequency and regulating b the detected frequency the amplitude o the signal modulatedca'rrier tov be transmitted.

l0. In a system for controllingthe transmission level between stations of a signaling system, the combination with a transmitting circuit at one station of the system lhaving a source of carrierY frequency, a

source of signal frequency, a modulator connected with both sources, a regulator tc govern the amplit-ude of the signal modulated wave, and meansto transmit the said signal modulated wave, of a receiving circuit at a second station ofthe system -having means to receive the said signal modulated wave, means to generate a control Wave,

means to transmit the said control wave, and` a receiving circuit at the said first mentioned station including means to receivethe said control wave, and means to elfectively apply the said control wave to the said regulator to produce variations ins amplitude of the signal modulated wave opposite in direction to the variations of the control Wave.

l1. In a system -for controlling the transmission level between stations of a signaling system, the combination with a transmitting circuit at one station of the system hav- .ing a source of carrier frequency, a source of signal frequency, a modulator connected with both sources, a regulator to govern the amplitude ofthe signal modulated Wave, and means to transmit the said signal modulated wave, ofa receiving circuit ata second station of the system including means to re` ceive the said signal modulated wave, means to generate a control wave, means to transmit the said control wave, and a receiving circuit at theffirst mentioned station having means to receive the said control wave, means to rectify. the control wave and means to effectively apply the rectified Wave to the said regulator to produce variations in the amplitude of the signal modulated. wave opposite in direction to the variations of the control wave.

l2. In a system for controlling the transmission level bet-ween stations of'a signaling syste/m, the `combination with a transmitting circuit at one station of the system having a source of signal modulated waves, a regulator to govern the amplitude of the said waves and Vmeans to Atransmit the said waves, of a receiving circuit at a second station of, the' system having .means to receive the signal modulated waves and an interfering wave, means to separate by frequency selection the said signal modulated waves and said interferinoI wave, means to amplify the signal modulated waves, means to rectify the interfering wave, means alsoat the said second station to generate and transmit a control wave, means connected with esA andl responsive to govern the amplitude of the control Wave as transmitted and also the degree of amplication produced by lthe said amplifier upon the received signal modulated Waves, and a receiving circuit atthe said first mentioned station having means to receive the said control Wave and to effectively apply the-said control Wave to the said regulator to produce variations in "the amplitude of the signal modulated Wave opposite in direction to the variations of the control Wave.

13. In a system for controlling the trans# mission level between stations of a signal-- ing system, the combination with a transmittingcircuit at one station of a system to the said rectifying means having a source of control Waves, of a ref I ceiving circult at the same station having' means to receive signal Wavesi and interferlng Waves7 means toA effectively separate said signal Waves and said interfering Waves by virtue` of their frequency difference," and means to employ said interfering Waves to vary the amplitude of the control wave as transmitted, the said variations being opposite in direction to the variations of the said RALPH it. POTTER. 

